Botanical products are becoming increasingly popular as alternatives to estrogen replacement therapy (ERT). Clinical evidence strongly supports an association between ERT and reduced risk of colon cancer in postmenopausal women. After follow-up of women taking combined estrogen-progestin replacement therapy, the Women's Health Initiative recently reported a 37% reduction in colorectal cancer cases compared to women on placebo. In another 20 published studies, half support an inverse association and another quarter shows a significant reduction in risk. Despite these findings, few investigations are underway to determine the mechanisms by which this preventive effect is achieved. Estrogenic effects are mediated by binding to a nuclear receptor and we determined that the second estrogen receptor subtype, ERbeta, is the subtype found in the colonic epithelium. Our studies also showed reduced expression of ERbeta mRNA in colon tumors compared to normal mucosa in female patients. Furthermore, following over-expression of ERbeta, human colon cancer cells displayed reduced proliferation rates and anchorage independent growth. These data imply that ERbeta could mediate the chemoprotective effects for ERT. The overall hypothesis being tested is that alternative ERT based on phytoestrogens with high ERbeta binding activity will reduce colon carcinogensis. Specific aims are proposed to test the following hypotheses: (1) Overexpression of ERbeta in human colon cancer cells will decrease tumorigenic phenotype in vitro (cells in culture) and in vivo (xenograph growth in mice) by interference of EGFR signaling pathways, and (2) A phytoestrogen-enriched diet will reduce experimentally induced aberrant crypt foci by decreasing proliferative activity in the colonic epithelium. The goal of this application is to define how activation of ERbeta-mediated responses can modulate human colon cancer cell growth and whether a red clover extract with ERbeta-selectivity can decrease carcinogen initiated colon tumorigenesis. If phytoestrogens show a chemopreventive effect on colon cancer development and the mechanism is related to ERbeta, these data would clearly expand our understanding of colon cancer and provide a new therapeutic strategy. The ability to pinpoint how an alternative ERT inhibits the adenoma-carcinoma sequence could also lead to more effective methods in preventing cancer recurrence in female patients, as well as developing better chemoprevention strategies in post-menopausal women. The clinical translational potential of this hypothesis lies in the ability of ERbeta positive epithelial cells to respond to phytoestrogens. These results would also suggest additional potential for chemoprevention strategies based on selective estrogen receptor (ERbeta) therapies.